A certain number of processes having as an object the preparation of polysulfided olefins for use as extreme-pressure additives for lubricants are disclosed in the prior art.
In particular, U.S. Pat. Nos. 3,471,404 and 3,697,499 disclose a process whose main steps are the following: (1) reaction of sulfur monochloride with an excess of an olefin having 2 to 5 carbon atoms, particularly isobutene, at a temperature from 20.degree. to 80.degree. C., so as to form an "adduct"; (2) reaction of the "adduct" of the first step with a sulfide of alkali metal (preferably sodium sulfide) and element sulfur, used in a ratio from 1.8 to 2.2 moles of metal sulfide per gram-atom of sulfur, the proportion of alkali metal being from 0.8 to 1.2 mole per mole of adduct, and the reaction being performed in the presence of an alcohol or of a hydro-alcoholic solvent, at reflux; and (3) reaction of the obtained product, containing 1 to 3% of chlorine, with an inorganic base in aqueous solution, at reflux, until the chlorine residual content of the product be lower than 0.5%.
It is stated in these prior art patents, that the sulfur content of the obtained products could be from 40 to 60% by weight. In fact, it is more often of 46% by weight. These products may be used as extreme-pressure additives for lubricating oils, transmission fluids or greases, the considered lubricating bases consisting of mineral oils and certain synthetic oils.
On the other hand, U.S. Pat. No. 4,204,969 discloses a rather similar process to prepare polysulfided olefins for use as extreme-pressure additives for lubricating oils. This process comprises the following main steps of:
(1) reacting, at about 30.degree.-100.degree. C., sulfur monochloride with a C.sub.3 -C.sub.6 aliphatic monoolefin (generally isobutene) preferably in the presence of a promoter consisting of a lower alcohol, so as to form an "adduct";
(2) reacting said "adduct" with sulfur and sodium sulfide (prepared for example from NaOH, NaHS and/or H.sub.2 S) in a proportion of 0.1 to 0.4 gram-atom of sulfur per mole of sodium sulfide, in hydroalcoholic medium, at a temperature from 50.degree. C. to reflux temperature; and recovering the obtained product without treatment by means of a base.
In the single example of this patent, the prepared product is stated to have a sulfur content of 49% by weight and a viscosity at 37.8.degree. C. (100.degree. F.) of 8.6 mm.sup.2 /s (cSt).
When it is desired, by using the processes of the prior art, to increase the sulfur content of the additives by increasing the proportion of element sulfur used in their preparation, as compared to the involved amount of the alkaline sulfide or hydrogenosulfide, the obtained products have no longer a sufficient solubility in synthetic lubricating oils (e.g. of the polyalphaolefin type) or even in mineral lubricating oils, to be used as extreme-pressure additives. Moreover, the kinematic viscosity of the so-obtained products is generally too high.
In the French Pat. No. 2,563,231, the applicant has already disclosed a process for the preparation of polysulfided olefins, defined mainly by the following steps of:
(1) reacting at least one compound selected from sulfur monochloride and sulfur dichloride with at least one monoolefin having 2 to 5 carbon atoms (generally isobutene), in a proportion from 1.5 to 2.5 moles of monoolefin per mole of sulfur monochloride and/or dichloride, thus forming an addition product or "adduct";
(2) contacting said "adduct" and at least one hydrocarbyl halide selected from C.sub.1 -C.sub.12 alkyl-, C.sub.5 -C.sub.12 cycloalkyl- or substituted cycloalkyl-, and C.sub.6 -C.sub.12 arylalkyl- or substituted arylalkyl-chlorides, bromides and iodides, the proportion of said hydrocarbyl halide corresponding to 2-40% of halogen gram-atoms in proportion to the total number of halogen gram-atoms of the assembly formed by said "adduct" and said hydrocarbyl halide (i.e. 0.015-0.55 gram-atom of halide per 100 g of "adduct"), with at least one sulfur compound selected from sulfides, hydrogenosulfides and polysulfides of alkali metals, ammonium or alkaline-earth metals, used in a proportion of about 0.4-0.8 mole per halogen gram-atom contained in the whole assembly formed by said "adduct" and said hydrocarbyl halide, and a proportion of element sulfur from 0 to 4 gram-atoms per mole of said sulfur compound, within a medium consisting of water or a mixture of water with aliphatic monoalcohol;
(3) heating the resultant mixture and, after separation in two phases, recovering the polysulfided olefin in the organic phase, and
(4) optionally treating the product obtained from step (3) with a basic compound such as an inorganic base.
This prior art patent indicates the so-prepared products are polysulfided olefins, whose sulfur content may reach generally up to 45-55% by weight and sometimes more (about 60% by weight). Their kinematic viscosity at 100.degree. C. varies in relation with their sulfur content. It may range from about 4 to 20 mm.sup.2 /s. Their halogen (mainly chlorine) content is generally lower than about 1% by weight and mostly than about 0.6% by weight.
The products whose preparation involves the use of the highest elemental sulfur proportions are conveniently used to form oils for metal working (cutting, shaping etc.). In this application there is the possibility of using polysulfided olefins prepared by using molar ratios of sulfur compound to element sulfur as low as 0.3/1, these products (of very high sulfur content) being still sufficiently soluble in base oils of mineral origin used in the lubricant compositions for metal working (e.g. 100 Neutral Solvent).
Here, the additive concentration is generally about 0.5-2% by weight.
In the application for the first addition certificate to the above-mentioned French Pat. No. 2,571,380, the applicant has extended the definition of the process for preparing polysulfided olefins to a process wherein, in step (2), the proportion of hydrocarbyl halide ranges from 40 to 70%, as halogen gram-atoms, with respect to the total number of halogen gram-atoms of the assembly formed by the "adduct" obtained in step (1) and said hydrocarbyl halide, and the proportion of element sulfur may reach up to 7 gram-atoms per mole of sulfur compound involved.
The process thus provides, particularly for the highest ratios "element sulfur gram-atoms/sulfur compound moles" (e.g. about from 3.3/1 to 7/1), products of very high sulfur content (up to about 60-65% by weight) and of still increased solubility in oils designed for metal working (e.g. of the 100 Neutral Solvent or naphthenic Spindle oil type).
The two above-mentioned specifications are coupled in particular in the European patent application No. 0,159,936.